Figure 1: Horse with sarcoid lesion before treatment. | Photo: Dr. Anna Hollis/Equine Disease Quarterly

Sarcoids are believed to be the most common skin tumor of the horse and affect about 2% of the worldwide population. Although they do not metastasize (spread to distant locations), they undoubtedly cause welfare concerns, especially in the ulcerated “fibroblastic” form. This may be through discomfort from the lesions themselves, from fly interference with the lesions, and also from administered treatments. In their most extreme forms, they can affect eyelid function and lead to secondary ulceration of the eye’s surface (Figure 1).

Clearly, they are not a “benign” lesion, despite their benign classification, and they should never be ignored. The presence of sarcoids also has financial implications not only due to the (sometimes very high) cost of treatment, but also due to reduced resale value.

There is compelling evidence that sarcoids are caused by a bovine papillomavirus, which is believed to be transmitted by flies, most likely from infected cattle, but possibly also from infected horses. It remains unclear exactly how the virus leads to neoplastic (cancerous) change, or why the virus is able to cause disease in more than one species. Interestingly, a (human) papillomavirus also is responsible for the vast majority of cases of cervical cancer and an increasingly large proportion of tumors of the head and neck in humans; clearly there is much to be learned about the implications of infection with papillomaviruses in all species